Field of the Invention
The field of the invention is the use of holographic optical elements to convert the information of a spectral distribution of light to another form. For convenience, the term "light" will be used to mean electromagnetic radiation of any and all wavelengths.
Many applications require discrimination or selection of wavelengths, but with different resolution requirements. For example a prism can be used as a low resolution spectrometer to separate visible light into its constituent colors. However other applications require isolation of narrow spectral lines to resolve a spectral shape.
One such application is a lidar system to measure wind velocities by aerosol and/or molecular backscatter. In a direct, detection Doppler lidar or incoherent lidar, the Doppler shift resulting in a pulse of narrowband laser light from scattering by aerosols or molecules is measured. A zero-wind reference spectrum of an outgoing laser beam is measured by the collection of light scattered from the outgoing optics. The reference spectrum and a backscattered laser light return signal pass through receiving optics. The reference spectrum is subtracted from the return signal to determine the Doppler shift. A high resolution spectroscopic device, typically a Fabry-Perot interferometer is used to detect the wavelength shifts.
The Fabry-Perot interferometer produces a circular interference spectrum or fringe pattern of equal area rings representing equal wavelength intervals, sharing a common axis, at the infinity focus of an objective lens system. Different types of image plane detectors have been created which attempt to match the circular pattern. Converting the pattern itself to fit linear detectors has also been accomplished.
One such image plane detector is described in "Image plane detector for the Dynamics Explorer Fabry-Perot interferometer," Timothy L. Killeen, B. C. Kennedy, P. B. Hays, D. A. Symanow, and D. H. Ceckowski, Applied Optics, Vol. 22, No. 22, Nov. 15, 1983, pp. 3503-3513. This device consisted of an S-20 photocathode, three microchannel plate electron multiplication stages, and an equal-area concentric-ring segmented anode to match the interference ring pattern. Another type of image plane detector was a multi-element detector of concentric rings of PIN photodiode material. (See U.S. Pat. No. 5,239,352 "Multiple Scattering Technique (MUST) Lidar," Luc R. Bissonnette, Issue Date Aug. 24, 1993 and "Multiple field of view lidar returns from atmospheric aerosols," D. L. Hutt, L. R. Bissonnette, and L. Durand, Applied Optics, Vol. 33, No. 12, Apr. 20, 1994, 2338-2348. The image plane detectors typically suffer from blurring of spot sizes and low quantum efficiency.
A different approach for converting the information in a Fabry-Perot fringe pattern to a more easily detectable form is described in U.S. Pat. No. 4,893,003, "Circle-to-Line Interferometer Optical System," Paul B. Hays, Issue Date: Jan. 9, 1990 and "Circle to line interferometer optical system," Paul B. Hays, Applied Optics, Vol. 29, No. 10, Apr. 10, 1990, 1482-1489. A 45 degree half angle internally reflecting cone segment is used to convert the circular Fabry-Perot interferometer fringe pattern into a linear pattern.
A small, high efficiency, compact, low cost device for obtaining the most information in a Fabry-Perot pattern while being compatible with linear arrays of detectors is desired. Being able to couple the pattern to solid state photodetectors as opposed to charge coupled device (CCD) detectors has the benefit of no cooling requirements when measuring atmospheric wind profiles. Also, solid state photodetectors can resolve the wavelength phase shift in the microsecond time interval needed for range resolved lidar measurements. In addition, a lidar system with multiple fields of view or, in other words, a very wide field of view is desired to avoid or lessen movement of the optical system in order to change zenith angle.